In general attempts have been made to prepare maleimido-conjugates of L-asparaginase from Escherichia coli and Erwinia carotovora. For example, the diazonium salt of p-aminophenyl maleimide was reacted with L-asparaginase from E. coli to form the diazophenyl maleimide adduct. Various conditions of coupling were studied to obtain optimum results in retention of enzyme activity and degree of derivatization. The purified enzyme conjugate is yellow in color and possesses absorption maxima at 280, 340 and 485 nm compared to 280 nm for the control enzyme indicating successful formation of the adduct. The extent of derivatization, as determined spectrally by reaction of the derivative with C14 cysteine, increased at higher pH values. Approximately 2 moles of reactive maleimide per mole of enzyme were obtained at pH 9.0 whereas only 0.2 mole of maleimide per mole of enzyme was evident at pH 7. Approximately 50% of the enzyme activity was retained under the most drastic reaction conditions. Efforts will be made to increase the extent of derivatiza-tion and the retention of enzyme activity. Also other derviatives of L-asparaginase will be prepared. For example we are beginning to work on the preparation of maleimidobenzoic acid adducts of L-asparaginase. Within the next year we hope to evaluate several derivatives in vivo in mice and/or rabbits for depression of tissue asparagine and for increased therapeutic value.